A variety of supplemental recovery techniques have been employed to increase the recovery of oil from subterranean formations. These techniques include thermal recovery methods, waterflooding and miscible flooding. The thermal recovery methods generally include steam injection, hot water injection and in-situ combustion.
Of the thermal recovery methods, in-situ combustion is showing increasing promise. In an in-situ combustion process, it normally involves injecting air under pressure into the oil-containing formation via an injection well to burn part of the formation oil and establish a combustion front that drives the rest of the oil which has been mobilized from the heat generated toward a spaced apart production well from which oil is recovered. Combustion or flue gases are produced along with the oil at the production well. These flue gases are not flammable due to their low heating value. A typical flue gas produced from an air driven in-situ combustion operation is about 18 BTU/SCF which is not flammable in air. The general practice is to add methane to the flue gas to raise its heating value and thus incinerate the resulting gas mixture.
A recent trend in in-situ combustion technology is to inject enriched air or pure oxygen instead of air, see U.S. Pat. No. 3,208,519 to T. V. Moore. The combustion gases from this process generally have higher heating values due to a higher carbon monoxide and methane concentration.
In the present invention, I propose an improved in-situ combustion process employing an oxygen-containing gas with a predetermined oxygen concentration to maximize the BTU value of the flue gas combined with separating carbon dioxide from the produced flue gas to further increase its heating value enabling the CO.sub.2 -free gas to be used as a fuel.